Method for fastening an accessory in a plastic fuel tank

ABSTRACT

Method for stake-fastening an accessory ( 4 ) into a plastic fuel tank, whereby:
         the accessory ( 4 ) is equipped with at least one orifice ( 5 ) which passes right through the accessory ( 4 );   at least some of the plastic of which the wall of the tank ( 1 ) is made is melted; and   some of the molten plastic is forced through the orifice ( 5 ) of the accessory without becoming detached from the remainder of the molten plastic;   the protruding molten plastic is given an appropriate shape ( 8 ) to obtain a self-formed plastic rivet, and is left to solidify;
 
the size and shape of the orifice ( 5 ) and/or of the solidified plastic ( 8 ) being such that the accessory ( 4 ) is mechanically fastened to the tank by at least some of the solidified plastic ( 8 ), the stake-fastening of the accessory ( 4 ) occurring at the time of manufacture of the tank ( 1 ) through the moulding of a split or at least two-part parison.

The present invention relates to a method for fastening an accessory ina plastic fuel tank.

Fuel tanks on board vehicles of various kinds generally have to meetimperviousness and permeability standards in relation to the type of usefor which they are designed and the environmental requirements that theyhave to satisfy. Both in Europe and throughout the world we arecurrently experiencing a considerable tightening of the requirementsconcerned with limiting the emissions of pollutants into the atmosphereand into the environment in general. The design of fuel tanks istherefore evolving quickly towards techniques capable of betterguaranteeing the imperviousness and safety under varying conditions ofuse. Furthermore, attempts are also being made to minimize the lossesoriginating from the pipes and various accessories associated with thetanks. One means sometimes used is to incorporate certain accessoriesand pipe work within the tanks, thus eliminating their interface withthe external surroundings. As a preference, these elements are fixed tothe internal wall of the tank and do not pass through it, so as to avoiddamaging the barrier layer often present in these tanks and so as toeliminate the risk of leakage associated with recourse to sealingdevices.

Thus, Patent Application WO 01/60592 in the name of the applicantcompany discloses a method of manufacturing a plastic fuel tank from twoshells formed by moulding, that are assembled, the moulding of at leastone shell being performed by compressing part of a sheet of plasticbetween a mould and a punch and by blowing the remaining part of thesheet in the region not compressed. The advantage of this method is thatit allows one or more accessories easily to be included inside the tank,preferably by welding onto the internal face of the tank, for thepermeability reasons mentioned hereinbelow. This fastening technique issimple but, unfortunately, limited to accessories having at least partof their basic surface made of the same plastic as the tank (generallyHDPE) or, at least, of a plastic compatible therewith. Now, numerousaccessories are based either on a metal or on a plastic different fromthat of the tank, having improved rigidity, improved dimensionalstability and improved impermeability to hydrocarbons, such aspolyacetal (and, in particular, polyoxymethylene or POM), polyamide,possibly glass-fibre-reinforced polyamide, polybutylene terephthalate(PBT) etc. An alternative fastening technique proposed in this patentapplication consists in at least partially coating the accessory withpart of the sheet at the time of its moulding. However, such a practiceuses up a great deal of material and sometimes requires significantmodifications to be made to the accessory. Furthermore, this techniquemay generate deformations and internal tensions as the skin of the tankcools.

It is therefore an object of the invention to provide a method thatallows an accessory based on a material different from that of the tankto be fastened inside a plastic fuel tank, and fastened quickly, easily,without consuming excessive amounts of material and without the risk ofdamaging the barrier layer or of creating excessive internal tensions.

The invention is based on the idea of benefiting from the fact that aparison is melted during its moulding, and. that it can be opened up(i.e. split or made up of two independent parts that can be parted fromone another) so that an accessory can be stake-fastened in it. Thisfastening technique is used in application U.S. Pat. No. 5,308,427, butin order to fasten an accessory in an existing (already manufactured)fuel tank that has to be locally heated (using hot gas or the injectionof molten material). As the fastening occurs after production, it is notpossible to benefit from a local flux of material. In consequence, thattechnique requires an external addition of material, with the inherentrisks of non-homogeneity. Furthermore, in the case of a multilayer tank(typically, made of high density polyethylene (HDPE)) with a layer ofEVOH (a copolymer of ethylene and partially hydrolysed vinyl acetate),it is necessary to be careful not to damage the continuity of thebarrier layer. It is therefore necessary to melt through the entirethickness locally. This heating step is lengthy and consumes power andfurthermore gives rise to deformation and local tensions.

The invention therefore relates to a method for stake-fastening anaccessory in a plastic fuel tank, whereby:

-   -   the accessory is equipped with at least one orifice which passes        right through the accessory;    -   at least some of the plastic of which the wall of the tank is        made is melted; and    -   some of the molten plastic is forced through the orifice of the        accessory without becoming detached from the remainder of the        molten plastic;    -   the protruding molten plastic is given an appropriate shape to        obtain a self-formed plastic rivet, and is left to solidify;        the size and shape of the orifice and/or of the solidified        plastic being such that the accessory is mechanically fastened        to the tank by at least some of the solidified plastic, the        stake-fastening of the accessory occurring at the time of        manufacture of the tank through the moulding of a split or at        least two-part parison.

The term “fuel tank” is intended to denote a sealed tank able to storefuel under diverse and varying environmental and usage conditions. Anexample of this tank is a tank fitted to motor vehicles.

The fuel tank produced according to the method according to theinvention is made of plastic, that is to say of a material comprising atleast one synthetic resin polymer.

All types of plastic may be suitable. Particularly suitable plasticscome from the thermoplastics category.

The term “thermoplastic” denotes any thermoplastic polymer, includingthermoplastic elastomers, as well as blends thereof. The term “polymer”denotes both homopolymers and copolymers (especially binary or ternarycopolymers). Examples of such copolymers are, non-limitingly, randomcopolymers, linear block copolymers, other block copolymers and graftcopolymers.

Any type of thermoplastic polymer or copolymer whose melting point isbelow the decomposition temperature is suitable. Syntheticthermoplastics that have a melting range spread over at least 10°Celsius are particularly suitable. Examples of such materials includethose that exhibit polydispersion in their molecular weight.

In particular, polyolefins, thermoplastic polyesters, polyketones,polyamides and copolymers thereof may be used. A blend of polymers orcopolymers may also be used, as may a blend of polymer materials withinorganic, organic and/or natural fillers such as, for example, butnon-limitingly, carbon, salts and other inorganic derivatives, naturalfibres or polymeric fibres. It is also possible to use multilayerstructures consisting of stacked layers bonded together comprising atleast one of the polymers or copolymers described above.

One polymer which is often used is polyethylene. Excellent results havebeen obtained with high density polyethylene (HDPE). As a preference,the tank also comprises a layer of a resin impermeable to the fuel, suchas EVOH for example (a copolymer of ethylene and partially hydrolysedvinyl acetate). Alternatively, it may be subjected to a surfacetreatment (fluoration or sulphonation) the purpose of which is to renderit impermeable to the fuel.

An “accessory” is intended to denote:

-   -   any object or functional device generally associated with the        fuel tank in its conventional mode of use or of operation and        which collaborates therewith in order to perform certain useful        functions; or    -   a support for one or several of such devices.

Non-limiting examples of such devices are: liquid pumps, level gauges,pipettes, tanks or baffles internal to the fuel tank, ventilationdevices, electronic units and stiffening bars.

According to one advantageous embodiment of the method according to theinvention, the accessory is in actual fact a preassembled structurecomprising a support and one or several identical or different deviceswhich are fixed by any suitable fastening means. Examples of these meansare clipping, screw-fastening, welding, etc. It is also advantageous forthe preassembled structure to bear means that allow additional devicesthat may be added on later to be attached. These means are also clippingdevices, tapped holes or threaded protrusions of circular shape to allowscrewing, surface regions suited to welding, etc. In the same vein, theaccessory may consist of a simple support comprising suitable means forthe later attachment of one or more devices. In other words, as apreference, the accessory comprises a support which is either equippedwith fastening means for one or several functional devices of the fueltank (and which may be the lid of the accessory), or bears one orseveral such devices directly. In this case, the orifice used forstake-fastening the accessory is preferably situated on the support.

According to the invention, the accessory comprises an orifice (hole)which passes right through its wall and allows the “staking” techniqueto be applied. This technique consists in forcing molten plastic throughthe orifice and causing it to protrude on the opposite side to the entryof said material into the orifice, while at the same time giving theprotruding plastic an appropriate shape such that it somewhat“self-forms” a plastic rivet (i.e. it forms the rivet in-situ frommolten plastic from the wall of the tank). This shape is generally thatof a plateau.

The orifice may have any shape. However, its cross section (in sectionparallel to the wall of the accessory) will generally not be constantbut will vary in such a way as to create a volume (intended to beoccupied by the molten plastic) of a suitable shape so as to create anon-removable mechanical connection once the said material hassolidified. Furthermore, the shape of the cross section (and itsvariation along a plane perpendicular to the wall of the accessory) willpreferably be tailored in such a way as to make it easier to force thematerial through the orifice, and for this to be done using anappropriate tool (insert).

It is also often advantageous from a technical standpoint to contrivefor the molten plastic not to entirely fill the orifice, and for this tobe achieved by means of an appropriate tool (counterform). This approachallows variations in volume of the plastic (shrinkage upon cooling,swelling upon contact with the fuel) to be accommodated withoutgenerating stresses. In this case, the plateau will preferably have amore or less oblong shape (whereas it is generally more or less circularin shape, as, incidentally, is the upper part of the orifice) orsemi-oblong shape (if the flow of material through the orifice has beenimpeded on just one side, something which may be advantageous becausethe shrinkage is typically greater than the swelling in the presence ofpetrol), the orifice then preferably also having a more or less oblongshape. The tools mentioned hereinabove (insert, counterform in themould) are preferably cooled so as to accelerate the actual fastening ofthe accessory and allow the rates of manufacture of integrated tanks(comprising at least one fixed accessory) to be increased.

In general, in the method according to the invention, recourse is had toseveral staking orifices. These are in fact somewhat pinpoint-like, i.e.small in size by comparison with the size of the accessory. Typically,their size is of the order of a few mm (10 mm or less).

An advantageous embodiment positions the staking orifices, for theoblong or semi-oblong versions, in the direction of the dimensionalchanges. These changes generally occur along lines which meet at a pointlocated at the centre of the various orifices. In the case of (more orless) oblong orifices, one way of achieving this is to arrange the saidorifices in such a way that the planes normal to the surface of theaccessory at the orifices and containing the longitudinal axes thereofintersect along the same axis (i.e. along one and the same straight linewhich kind of constitutes an axis of symmetry of the accessory). Whenthe longitudinal axes of the various orifices are in one and the sameplane, this amounts to saying that these axes meet at a point more orless at their centre. This approach allows the accessory not to bedeformed or shifted excessively with respect to the centre of thevarious orifices.

In the method according to the invention, the plastic of which the wallof the tank is made at the site intended for fastening the accessory ismelted (which means to say in fact that it is truly subjected to fusionin the case of a semicrystalline polymer such as HDPE, but in fact meansto say plasticized/softened in the case of amorphous polymers). Thepasty material is then forced through the orifice of the accessorywithout detaching from the wall of the tank, and to solidify there. Asalready mentioned earlier, the size and shape of the orifice and/or ofthe solidified plastic are such that the accessory is mechanicallyfastened to the tank by at least some of the solidified plastic.

The accessory is fastened by the method according to the invention atthe time of manufacture of the tank by moulding. This approach can beachieved in practice by virtue of the fact that moulding takes placestarting from a split or at least two-part parison. In particular,advantageously, the stake-fastening occurs at the time of moulding ofthe tank by:

-   -   thermoforming sheets;    -   compressing/blow-moulding sheets (as described in the        aforementioned application WO 01/60592, the content of which for        this purpose is also incorporated into this application);    -   blow-moulding a parison comprising at least one cut-out (as        described in Patent Application EP 1110697, the content of which        for this purpose is incorporated by reference into this        application).

When the tank is produced by thermoforming sheets (a technique thatessentially employs a mould consisting of two moulding cavities), thestake-fastening may be performed by a robot which introduces theaccessory between the two parts of the mould in which the sheets areinserted and pushes it against these sheets in order to perform thestake-fastening. In this case, the tooling mentioned hereinabove(insert,counterform in the mould) can be incorporated into the robot arm.

When the tank is produced by blow-moulding or bycompression/blow-moulding (techniques both of which simultaneously use acore and an actual mould proper consisting of moulding cavities), thestake-fastening is preferably performed with the aid of an insert on themould side (so as to force the material through the orifice) and acounterform actuated by a ram on the core side (so as to deform thematerial which has protruded through the orifice).

A method of manufacturing the tank and of fastening the accessory byblow-moulding (in a mould comprising a core and a moulding cavity) whichuses such accessories generally runs as follows:

-   -   positioning the accessory on the core;    -   initial closing of the mould (moulding cavities brought around        the core);    -   pressing a split or at least two-part parison (or wall of a fuel        tank made of plastic in the molten state) onto the moulding        cavities of the mould (by blowing through the core and applying        suction behind the moulding cavities);    -   advancing a hydraulic ram which is fastened to the core in order        to perform the staking;    -   pricking the parison using a needle;    -   degassing, opening the mould, removing the core, closing the        mould again and blowing in a conventional manner.

During this method, there is of course a retraction of the hydraulic ramwhich may occur before, during and/or after degassing. There is also adevice preventing the slot or the edges of the parts of the parison fromwelding together during the initial closing of the mould (in order toperform the stake-fastening). To achieve this, the moulding cavities ofthe mould are advantageously provided with a heat regulating device thatallows this region to be heated during the steps involved with theinitial closing of the mould. Such a device is described for example inapplication FR 04.13407 in the name of the applicant company, thecontent of which for this purpose is incorporated by reference into thisapplication.

In this method, the insert preferably bears a protrusion in the shape ofa spike making an angle preferably of between 30 and 60°, or even 40 and50°, and ideally of about 45°, thus encouraging the material of whichthe wall is made to flow through the orifice. Any abrupt transition inangle may be provided with a rounded radius so as to optimize the flowduring the staking phase while at the same time preventing the barrierlayer of the tank, if there is one, from rupturing.

With a view to strengthening the mechanical connection between theaccessory and the wall of the tank, the counterform preferably appliespressure via the hydraulic ram not only to the accessory itself but alsoat least partially to the molten plastic of the wall of the tanksurrounding the accessory to encourage its compaction.

The present invention also relates to a plastic fuel tank equipped withat least one (support for) accessory in direct contact with part of thewall of the tank and being equipped with several orifices of more orless oblong shape in which and over which plastic identical to that ofthe wall of the tank is present, these orifices being arranged in such away that the planes normal to the surface of the accessory at theorifices and comprising the longitudinal axes of the latter intersect onone and the same axis.

In the context of the invention, the accessory may be based on amaterial different from the wall of the tank. Advantageously, theaccessory is based on at least one material compatible or compatibilizedwith that of the wall of the tank (for example PA compatibilized withHDPE particular by a suitable surface treatment or alternatively byovermoulding with an adhesive), which allows a strengthened mechanicalfastening and imperviousness through chemical bonding between the twomaterials when the accessory comes into contact with the molten materialof the tank to be fastened to the latter. Alternatively, the accessorymay be made of two materials, one offering dimensional stability and theother permitting adhesion between the accessory and the wall of thetank. For example, an accessory made of a material other than HDPE (forexample made of POM) can be overmoulded with HDPE.

The purpose of FIGS. 1 to 4 is to illustrate certain concrete aspects ofthe invention without in any way wishing to limit its scope.

FIGS. 1 and 2 each depict a section (on a plane perpendicular to thesurface of the tank at this point) through a tank to which an accessoryis in the process of being stake-fastened, the said tank being in theprocess of being manufactured by moulding but the section confiningitself to the vicinity of a staking point of given geometry. FIGS. 3 and4 depict a view of the internal surface of the finished tank (bearingthe accessory) seen in section (on planes 1-1′ and 2-2′ respectively) ofFIGS. 1 and 2.

FIGS. 1 and 2 depict a fraction of the wall of a fuel tank (1)comprising two layers of HDPE (2,2′) one on each side of a layer of EVOH(3), the said tank being in the process of being manufactured bymoulding a two-part parison. accessory (4) equipped with at least oneorifice (5) is in the process of being stake-fastened to this wall (1).Constituent material of the wall of the tank (1) has been forced throughthis orifice (5) by means of an insert (6) fixed in the mould and hasbeen. deformed on the inside of the tank by a counterform (7) operatedby a ram (not depicted but acting in the direction of the arrow) so asto form a kind of plateau (8) which overhangs the orifice (5) andprevents the accessory (4) from being removed without tearing/destroyingthe plateau (8). To these ends, the plateau (8) has dimensions tailoredto the desired mechanical strength for the fastening of the accessorybefore and after contact with a fuel.

The insert (6) and the counterform (7) are cooled in order to acceleratethe setting of the molten plastic around the orifice (5). The insert (6)bears a protrusion in the shape of a spike making an angle of about 45°(see FIG. 1), encouraging the constituent material of the wall (1) toflow through the orifice (5). In FIG. 2, the abrupt transitions in anglehave been given a rounded radius to optimize the flow during thestake-fastening phase while at the same time avoiding rupturing thelayer of EVOH.

In the case illustrated in FIG. 1, the plateau (8) is circular andplastic occupies the entire orifice (5).

In the case illustrated in FIG. 2, the circular plateau (8) has beendeformed (rendered oblong) by an appropriate relief (9) on thecounterform (7), this being so as to absorb the dimensional changesintroduced by shrinkage during cooling of the tank and expansion byswelling upon contact with the fuel.

In order to ensure good filling of this plateau (8), the counterform (7)has been designed to overhang the accessory (4) (see part (10) in thefigure) and press against the wall of the tank (1) in the vicinity ofthe accessory (4). This approach makes it possible to increase theinternal pressure at the plateau (8) and therefore improve thecompacting in the plateau (8).

The respective shapes of the plateaus (8) in the two cases areillustrated in FIGS. 3 and 4 respectively, in an overall view of theinterior surface of the tank bearing the accessory. In FIG. 3, theorifice (5), which is not visible, is circular and in FIG. 4, theorifice is oblong.

The accessory (4) illustrated in these figures comprises a support (4′)which allows for ease of staking (illustrated in the form of 3-pointstaking in these figures) and the accessory proper (4″) secured to it.It can be seen in FIG. 4 that the axes (see dotted lines) of the oblongorifices (5) meet at the centre of the support of the accessory (4′)which is more or less flat.

1-10. (canceled) 11: A plastic fuel tank equipped with at least oneaccessory or support for an accessory, wherein the accessory or supportfor an accessory in direct contact with part of the wall of the tank andincludes several orifices of oblong shape in which plastic identical tothat of the wall of the tank is present, these orifices being arrangedin such a way that the planes normal to the surface of the accessory atthe orifices and comprising the longitudinal axes of the latterintersect on one and the same axis. 12-14. (canceled)